Re: [PATCH v3 19/35] Documentation/x86: Document the new attack vector controls

Josh Poimboeuf
2025-02-11T16:44:00Z

On Wed, Jan 08, 2025 at 02:24:59PM -0600, David Kaplan wrote:
> +Cross-Thread
> +^^^^^^^^^^^^
> +
> +The cross-thread attack vector involves a malicious userspace program or
> +malicious VM either observing or attempting to influence the behavior of code
> +running on the SMT sibling thread in order to exfiltrate data.
> +
> +Many cross-thread attacks can only be mitigated if SMT is disabled, which will
> +result in reduced CPU core count and reduced performance. Enabling mitigations
> +for the cross-thread attack vector may result in SMT being disabled, depending
> +on the CPU vulnerabilities detected.
> +
> +*mitigate_cross_thread defaults to 'off'*

How does STIBP fit into this? It's a cross-thread mitigation, but it's
much cheaper than, say, disabling SMT.

The default is generally to enable STIBP where applicable, but *not* to
disable SMT.

--
Josh